Render in Alpha Channel

Good afternoon!

I was rendering some comps on ME using the option "Render alpha channel" so I can use them as mattes. However, I have some comps with footage on black screen, like smoke and fire using "screen" or "add" blending modes and instead of the  isolated subjects I get a full white screen on this footages when I select to render alpha channel. I changed the blending mode to "normal" and still, no subject. My solution was to add a green solid to the main comp as background and use keylight instead of matte, but the results aren't very good. I'm working on a project for kids, with rainbows with glow effect and the background has lots of different colors, so chroma key isn't the ideal solution, I imagine. Is there a better solution? How can I render the alpha channel with these footages?